CS 1

MIDTERm

Java Identifiers

name used for a variable, class, method, or other program element.

• Can contain letters, digits, _, and $.

• Cannot start with a digit.

• Cannot contain spaces.

• Java is case-sensitive: total, Total, and TOTAL are different.

• Class names conventionally use PascalCase, such as CoinFlip.

• Constants conventionally use uppercase, such as MAXIMUM.

• Reserved words have special meanings and cannot be used as identifiers, such as class, public, and int.

reserved words

• abstract

• assert

• boolean

• break

• byte

• case

• catch

• char

• class

• const

• continue

• default

• do

• double

• else

• enum

• extends

• false

• final

• finally

• float

• for

• goto

• if

• implements

• import

• instanceof

• int

• interface

• long

• native

• new

• null

• package

• private

• protected

• public

• return

• short

• static

• strictfp

• super

• switch

• synchronized

• this

• throw

• throws

• transient

• true

• try

• Void

• Volatile

• While

Primitive data types and variables

store simple values.

Type	Stores	Example
int	Whole numbers	int age = 18;
double	Decimal numbers	double price = 4.99;
char	One character	char grade = 'A';
boolean	true or false	boolean passed = true;

Other Java primitive types include byte, short, long, and float.

Declaring a variable:

data type then name ex) int score;

Initializing a variable:

int score = 10;

gives it its first value.

Assignment:

stores a value in a variable

Primitive variable:

holds a value, such as int x = 5;.

Reference variable:

holds a reference to an object, such as a String or a Scanner.

Assignment statements: the two steps

int x = 5;
x = x + 3;

For x = x + 3;, Java does two things:

1. Evaluate the right side: 5 + 3 = 8.

2. Store 8 in x, replacing the old value.

The final value of x is 8.

Remember: The = sign in Java means assignment, not mathematical equality.

Expressions and operator precedence

to determine what to calculate first.

Common order:

1. Parentheses ()

2. Multiplication, division, remainder: *, /, %

3. Addition and subtraction: +, -

4. Assignment: =

Operators with the same precedence are generally evaluated from left to right.

Example:

int result = 4 + 3 * 2;

Multiplication first: \(3 \times 2 = 6\).

Then addition: \(4 + 6 = 10\).

So result equals 10.

Integer arithmetic vs. floating-point arithmetic

Integer division drops the fractional part when both operands are integers.

System.out.println(7 / 2);      // 3
System.out.println(7.0 / 2);    // 3.5
System.out.println(7 % 2);      // 1

• / divides.

• % gives the remainder.

• int / int produces integer division.

• Using a double operand produces floating-point division.

This is one of the most important things to practice for code-tracing questions.

Expression trees

An expression tree shows the order in which an expression is evaluated.

Example:

4 + 3 * 2

The multiplication happens first, so * is the root's right child.

+

4

*

3

2

The tree represents 4 + (3 * 2), which equals 10.

CPU

Executes instructions and performs calculations.

Main storage (memory/RAM):

• Holds data and instructions while programs run.

Secondary storage:

Stores files and programs long-term, such as on an SSD or hard drive.

Byte:

• A group of 8 bits.

Bit:

A binary digit, either 0 or 1.

Number of patterns with N bits

Each bit has two possibilities, so:

\[ \boxed{2^N\text{ patterns}} \]

Bits	Number of patterns
1	2
2	4
3	8
4	16
8	256

Java program structure and compilation

A basic Java program:

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello!");
    }
}

Know what each part means:

• public class Hello: declares a class.

• main: the starting point of the program.

• System.out.println(): prints output and moves to a new line.

• { }: mark the beginning and end of blocks.

• //: starts a single-line comment.

• /* ... */: marks a block comment.

How Java runs

Source file — Hello.java

Compiler — javac

Bytecode file — Hello.class

Java Virtual Machine (JVM) — executes the bytecode

The source file contains the code you write. The compiler translates it into bytecode, and the JVM runs that bytecode.

string literal

text enclosed in double quotation marks.

System.out.println("Hello");

concatenation.

The + operator can join strings together.

System.out.println("Hello " + "there");

Increment and decrement operators

• x++ increments x by 1.

• x-- decrements x by 1.

• x += 3 adds 3 to x.

• x -= 2 subtracts 2 from x.

Know the difference between prefix and postfix:

int x = 5;
int a = x++;

After these statements, a is 5 and x is 6.

int x = 5;
int a = ++x;

Now a is 6 and x is 6.

Postfix uses the old value first. Prefix increments first.

Widening

Converts a value to a type that can represent a broader range of values, commonly without losing information.

int x = 5;
double y = x;

Here, y becomes 5.0

Narrowing

Converts to a type with a more limited representation and may lose information.

double price = 4.9;
int whole = (int) price;

whole becomes 4, because the decimal part is discarded.

Type promotion

Java may promote smaller numeric types during calculations.

int x = 5;
double result = x / 2.0;

The result is 2.5 because one operand is a double.

Scanner input

import java.util.Scanner;

Create a Scanner:

Scanner scan = new Scanner(System.in);

Method	What it reads	Example
nextLine()	A whole line of text	String name = scan.nextLine();
nextInt()	An integer	int age = scan.nextInt();
nextDouble()	A decimal or other double value	double price = scan.nextDouble();

import java.util.Scanner;

public class Addition {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        System.out.print("Enter a number: ");
        double x = scan.nextDouble();

        System.out.print("Enter another number: ");
        double y = scan.nextDouble();

        System.out.println("Sum = " + (x + y));
    }
}

String methods

Method	Purpose	Example result
length()	Returns the number of characters	word.length() → 5
charAt(index)	Returns the character at an index	word.charAt(1) → 'e'
toUpperCase()	Returns an uppercase version	word.toUpperCase() → "HELLO"

Remember: String indexing begins at 0.

For "Hello":

Character	H	e	l	l	o
Index	0	1	2	3	4

Math class

Method or constant	Meaning	Example
Math.sqrt(x)	Square root	Math.sqrt(25) → 5.0
Math.random()	Random number greater than or equal to 0 and less than 1	Math.random()
Math.PI	Approximation of pi	Math.PI

Example:

double radius = 3.0;
double area = Math.PI * radius * radius;
System.out.println(area);

Boolean expressions and relational operators

A Boolean expression evaluates to either true or false.

Operator	Meaning	Example
<	Less than	3 < 5 → true
>	Greater than	3 > 5 → false
<=	Less than or equal to	5 <= 5 → true
>=	Greater than or equal to	6 >= 5 → true
==	Equal to	5 == 5 → true
!=	Not equal to	5 != 3 → true

Don't confuse = with ==:

• = assigns a value.

• == compares values.

Logical operators

| Operator | Meaning | Example | |---|---|---|

| && | AND: both conditions must be true | true && false → false |

| || | OR: at least one condition must be true | true || false → true |

| ! | NOT: reverses a Boolean value | !true → false |

Truth tables

| A | B | A && B | A || B |

|---|---|---|---|

| true | true | true | true

| | true | false | false | true |

| false | true | false | true |

| false | false | false | false |

Simple and nested if statements

A simple if statement executes a block when a condition is true.

if (age >= 18) {
    System.out.println("Adult");
}

An if-else statement chooses between two paths:

if (score >= 60) {
    System.out.println("Pass");
} else {
    System.out.println("Fail");
}

A nested if is an if statement inside another if or else block.

if (score >= 60) {
    if (score >= 90) {
        System.out.println("Excellent");
    } else {
        System.out.println("Pass");
    }
} else {
    System.out.println("Fail");
}

Trace the conditions in order to determine which output appears.

Block statements

a group of statements enclosed in braces { }.

if (x > 0) {
    System.out.println("Positive");
    System.out.println(x);
}

Both statements belong to the if block. They execute only when x > 0.

Instance variable:

Belongs to an object and stores that object's data.

Local variable:

Declared inside a method or block and used there.

Parameter:

• A variable in a method declaration that receives a value when the method is called.

• 
  

Garbage collection:

Java automatically reclaims memory used by objects that are no longer reachable by the program.

Object references and aliases

A reference variable refers to an object. Multiple variables can refer to the same object; these variables are called aliases.

For example:

Coin first = new Coin();
Coin second = first;

Both first and second refer to the same Coin object. They are not two separate coins.

If the object is modified through one reference, the change can be seen through the other reference.

Important distinction: Assigning one primitive variable to another copies its value. Assigning one object reference to another copies the reference, so both may point to the same object.

Comparing Strings

For the exam, understand the difference between comparing String objects by reference and comparing their contents.

String a = new String("hello");
String b = new String("hello");

• a == b checks whether the references point to the same object. Here, the result is false.

• a.equals(b) checks whether the text contents match. Here, the result is true.

Use .equals() when you want to compare the text of two strings.

Writing a program that uses a formula

Your exam topics include writing programs that input numbers and use them in a formula.

A typical program follows this pattern:

1. Import Scanner.

2. Declare the class and main method.

3. Ask the user for input.

4. Read the input into variables.

5. Calculate the result.

6. Print the result.

Example: calculate the area of a rectangle.

import java.util.Scanner;

public class Rectangle {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        System.out.print("Enter length: ");
        double length = scan.nextDouble();

        System.out.print("Enter width: ");
        double width = scan.nextDouble();

        double area = length * width;

        System.out.println("Area = " + area);
    }
}

Defining and using classes

Your instructor lists Coin.java and CoinFlip.java as programs to review.

A class defines the data and behavior of a type of object. A program can create an object from that class and use it.

For example:

Coin myCoin = new Coin();

• Coin is the class name.

• myCoin is the reference variable.

• new Coin() creates a new object.

Review how a class is defined, how its instance variables work, and how another program uses the class.